Semiconductor structure including mixed rare earth oxide formed on silicon

ABSTRACT

A method (and resultant structure) of forming a semiconductor structure, includes forming a mixed rare earth oxide on silicon. The mixed rare earth oxide is lattice-matched to silicon.

RELATED APPLICATIONS

This Application is a Divisional Application of U.S. patent applicationSer. No. 10/998,840, filed on Nov. 30, 2004, which was a DivisionalApplication of U.S. patent application Ser. No. 09/898,039, (Now U.S.Pat. No. 6,852,575) filed on Jul. 5, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to an insulator, and moreparticularly to a lattice-matched epitaxial insulator structure formedon silicon, and arbitrarily grown, lattice-matched epitaxialinsulator-silicon structures grown on top of each other.

2. Description of the Related Art

Prior to the present invention, there has not been any lattice-matchedepitaxial insulator structure formed on silicon where the latticeconstant of the oxide can be varied arbitrarily so that it is equal to,or an integral multiple of the lattice constant of silicon. However,such lattice-matched epitaxial insulator structures on Si are needed forvarious reasons.

Firstly, such insulators can be used as gate dielectrics for Sicomplementary metal oxide semiconductor (CMOS) transistors with the viewthat an epitaxial structure will be less defective. Such epitaxialstructures based on SrTiO₃-type pervoskite structures have been grown asdescribed in “Crystalline Oxides on Silicon: The First Five Monolayers”,Rodney A. McKee et al., Physical Review Letters, Volume 81, Number 14,Oct. 5, 1998, pp. 3014-3017.

However, these structures have a lattice mismatch that is about 2% offfrom that of Si. Such structures can also be made with Y₂O₃ but thelattice mismatch is about 2.5%.

Secondly, such insulators can be used for fully epitaxialSi/insulator/Si epitaxial structures. There have been no prior reportsof successful growth of Si/oxide/Si epitaxial structure with a flatinterfacial and surface profile. These structures can be used for avariety of different applications such as, for example,silicon-on-insulator (SOI) structures for transistors, double-gated FETstructures, and novel optical devices.

Thus, prior to the invention, gate dielectrics/insulators have beenprovided that are epitaxial, but not lattice-matched. However, thesedielectrics/insulators are still problematic as lattice mismatch induceddefects are created in the devices (e.g., CMOS FET) incorporating suchstructures. These defects act as traps and affect the turn-on of thedevice (transistor), as well as the stability and mobility of thedevice.

In addition, prior to the present invention, no Si substrate/epitaxialoxide/epitaxial silicon structures have been grown that have smooth anduniform surfaces and interfaces. There has been one report (e.g., see“Epitaxial Ceo₂ on Silicon Substrates and the Potential of Si/ceo2/sifor SOI Structures”, A. H. Morshed et al. Mat. Res. Soc. Symp. V474,339(1197)) of attempting to grow epitaxial Si films on CeO₂ (ceriumoxide). However, the Si growth profile was rough and three dimensionaland the silicon was not completely epitaxial in nature.

Thus, prior to the present invention, there has not been anylattice-matched epitaxial insulator structure formed on silicon which issubstantially defect-free, nor has the advantages of such a structurebeen recognized prior to the present invention.

SUMMARY OF THE INVENTION

In view of the foregoing and other problems, drawbacks, anddisadvantages of the conventional methods and structures, an object ofthe present invention is to provide a lattice-matched epitaxialinsulator structure formed on silicon and a method of forming the same.

In a first aspect of the present invention, a method (and resultantstructure) of forming a semiconductor structure, includes forming amixed rare earth oxide on silicon. The mixed rare earth oxide islattice-matched to silicon.

With the unique and unobvious aspects of the invention, alattice-matched epitaxial insulator structure is formed on silicon.These structures can be used for a variety of different applicationssuch as, for example, silicon-on-insulator structures for transistors,double-gated FET structures, and novel optical devices.

The inventive compound has a lattice constant which is preferably twicethat of silicon, and thus it is a multiple such that everything “fits”.The insulator also possessed a high band-gap (75 eV), a high dielectricconstant (>10) and a low electrical current leakage. As a result,various band gap engineered thin film heterostructures with silicon maybe conceived. Some of these devices have been mentioned above, but arere-emphasized below.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other purposes, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 illustrates a flowchart of the method 100 according to thepresent invention;

FIG. 2A illustrates a structure 200A of the invention in its most basicform;

FIG. 2B illustrates a structure 200B formed by the method 100 of thepresent invention; and

FIG. 3 illustrates another structure 300 of the method of the presentinvention including a multi-layer stack structure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Referring now to the drawings, and more particularly to FIGS. 1-3, thereare shown preferred embodiments of the method and structures 10according to the present invention.

Prior to turning to the details of the invention, it is noted that thepresent invention has provided a new material, (La,Y)₂O₃ that can beclosely matched to the Si lattice for epitaxial growth.

That is, Yttrium oxide is a well-known oxide which has a cubic bixbyite(e.g., “bixbyite” refers to a specific cubic, crystallographicstructure) structure with a lattice constant of 1.06 nm, which is about2.4% smaller than two times the lattice constant of silicon. As aresult, when one grows thin films of Y₂O₃ on silicon, the deposition hasepitaxial characteristics but is not of good quality (e.g., defects arecreated).

Further, lanthanum oxide (La₂O₃) is a well-known oxide which has alarger La to O bond length, since La is a larger atom. However, undernormal circumstances La₂O₃ crystallizes with a hexagonal structure andcannot be lattice-matched to silicon.

There has been reported a cubic bixbyite La₂O₃ phase, and it has alattice constant of 1.138 nm. However, it is not a stable phase undernormal pressure and room temperature.

Thus, these known compounds are problematic. The present inventionprovides a new compound which overcomes the problems of these and othercompounds, as discussed below.

That is, in the present invention, a new, metastable (La_(x)(Y_(I-X))₂O₃alloy has been created. This alloy, at around x=0.3, will be perfectlylattice-matched to silicon and will crystallize in the cubic structure.That is, in its most basic form, as shown in the structure 200A of FIG.2A, includes the novel compound 230 grown on a silicon substrate 210, asdescribed in further detail below.

In spite of the tendency for La₂O₃ to crystallize hexagonally, the cubicmodification occurs due to crystal structure stabilization from theY-containing oxide and epitaxial stabilization from the Si substrate. Atx=0.33, the lattice constant of the alloy will be 1.086 nm (i.e., twicethat of silicon). This follows from a linear interpolation between theLa₂O₃ lattice constant of 1.138 nm, the Y₂O₃ lattice constant of 1.06nm, and using Vegord's law.

The present inventors have demonstrated that by straightforwardmolecular beam epitaxy (MBE), an epitaxial film of good quality may begrown on silicon by depositing (La_(x)Y_(1-X))₂O₃. This shows bright,reconstructed reflection high energy electron diffraction patternstypically observed in good epitaxial growth. X-ray diffraction of suchlayers also indicate that they are epitaxial, single crystal in nature,and of very high quality. It is noted that the invention does not relyon MBE, but instead can employ other types of growth techniques.

Second Embodiment

Further, to the compound described in the first embodiment, in anotheraspect of the invention, the present inventors have also discoveredthat, after growing the epitaxial (La,Y)₂O₃ layer on silicon (e.g.,described below in further detail and shown in step 140 of FIG. 1), anepitaxial Si layer can be further grown on top of this epitaxial(La,Y)₂O₃ layer. Such epitaxial growth can be performed by MBE. This isagain clearly observed from in-situ reflection high energy electrondiffraction patterns. These patterns indicate that a smooth,reconstructed silicon surface may be obtained. This is the first time asilicon/oxide/silicon structure has been grown epitaxially.

Generally, it has been very difficult to grow silicon on the oxide, astypically it is amorphous and when silicon is grown it tends to becomepolycrystalline. Thus, the SOI structures of the invention are highlyadvantageous.

Example

Turning to FIG. 1, the method 100 of the invention will be describedhereinbelow.

First, in step 110, a wafer was prepared. That is, a clean Si wafer wastaken and dipped in a hydrofluoric acid solution in order to produce ahydrogen-passivated surface. The wafer orientation is preferably <111>,but it can also be <100> or <110>. The wafer may have a thickness withina range of about 1 μm to about 10000, uμ. However, it is noted that anythickness may be used, as the thickness of the wafer is not relevant tothe invention. Alternatively, one Si wafer may not be hydrogenpassivated and can retain a thin (2 nm or less) silicon dioxide layer onits surface.

Then, the wafer was loaded into a vacuum deposition system and heated toabout 600° C. in order to desorb the H from the surface and clean it up.It is also possible to heat the wafer to 10001° C. to clean it further,but such a step is not critical.

Following preparation of the Si surface, in step 120, an epitaxial Sibuffer was grown on top of the Si wafer resulting in a clean and smoothsurface. However, this step is not essential for the process (e.g., theSi buffer layer is optional).

If the silicon buffer layer is grown, then the starting surface shouldbe a clean, oxide-free surface. On the other hand, if the La,Y oxide isto be deposited, then it could be deposited directly on top of the thin(<2 nm) silicon oxide, or on an oxide-free silicon surface. In thisexample of the present invention, the vacuum chamber for deposition wasa standard molecular beam epitaxy chamber. The buffer layer may have athickness within a range of about 0.5 nm to about 1 micron or more.

In step 130, the (La,Y)₂O₃ (or another mixed rare earth oxide asmentioned below) is now ready for deposition. La and Y are heated usingstandard high temperature commercial effusion cells. Typical La and Ycell operation temperatures are 1300-1700 C. Depending upon the specificcell temperature, the element fluxes can be controlled (as is typical ina standard molecular beam deposition process) and thereby the relativecomposition of La:Y can be controlled in the deposited film. Oxygen isprovided by a molecular oxygen beam. Alternatively, radio frequency (RF)excited atomic oxygen can be provided as well, or in place of molecularoxygen.

With the La and Y cells raised to the appropriate temperature, and anoxygen flow rate of about 1-3 sccm, so that the chamber pressure is inabout the 1 E-5 to 1 E-4 range, the epitaxial growth of (La,Y)₂O₃ wascommenced by opening the La and Y shutters with the substrate facing allthree sources (e.g., La, Y, and O). The substrate temperature istypically about 650 C, but can be anywhere in the 400-800 C range.Epitaxial growth of (La,Y)₂O₃ now occurred. Films have been grown withcompositions from 0<x<0.5 for the compound (La_(x)Y_(1-x).X)₂O₃. Thelattice matching occurs at around x=0.3. Remarkably, single crystalfilms will grow even through a thin S₁O₂ layer is on the Si surface

When the Si wafer orientation was (001), the LaY-oxide was grown withits (011) direction normal to the film surface. When the Si waferorientation was (011), the LaY-oxide was grown with its (111) directionnormal. When the Si wafer orientation was (111), the LaY-oxideorientation was normal to the film surface was (111).

Therefore, in the first case, one obtains a two domain epitaxial growth,and in the other two cases, it is possible to get true epitaxial growth.

For epitaxial deposition on Si(111), near-lattice matched LaY-oxides maybe grown with a (222) x-ray diffraction full width at half maxima ofabout 130 arc seconds which indicates the high quality nature of thegrowth. The inventive compound (La_(x)Y_(1-x).X)₂O₃ may have a thicknesswithin a range of about 0.5 nm to more than 1 micron. Again, thethickness thereof is not relevant to the invention.

Following deposition of the oxide, the vacuum chamber was pumped down toabout 10⁻⁹ torr to reduce background oxygen pressures.

Then, in step 140, epitaxial silicon was deposited onto the oxidesurface by keeping the substrate at 650-700° C. and evaporating the Sifrom an electron beam source. The silicon was deposited epitaxially asclearly observed from reflection high energy electron diffractionimages. The epitaxial silicon layer may have a thickness within a rangeof about 0.5 nm to about more than 1 μm. Again, the thickness is notrelevant.

Thus, the method 100 is completed, and the completed structure 200B isas shown in FIG. 2B.

That is, in FIG. 2B, silicon wafer 210 has the optional buffer layer 220formed thereon. Then, the inventive compound (La_(x)Y_(1-x))₂O₃ 230 isgrown on the buffer layer 220 (or directly on the wafer 210), and a topsilicon layer 240 is deposited on the inventive compound(La_(x)Y_(1-X))₂O₃ 230. Such a structure would be a silicon-on-insulatorstructure. It is noted that the invention in its most basic form is theinventive compound 230 formed directly on the silicon wafer 210, asshown in FIG. 2A.

It is noted that the invention can be advantageously used to buildmultilayer stacks. That is, a stack could be built having a plurality oflayers.

For example, FIG. 3 illustrates a structure 300 similar to that of FIG.2B but in which an additional oxide layer 330 (which can havesubstantially or identically the same constituents as oxide layer 230,or in which a compound different from that of 230 can be used asdescribed below) has been grown on top of the silicon layer 240, andthen an additional silicon layer (not illustrated) would be formed, andso forth. It is noted that the layers of the figures are not necessarilydrawn to scale.

In the structure of FIG. 3, a Silicon quantum well (240) is formed, ifthe thickness of well 240 is less than about 20 nm. This quantum wellcan be the building block for resonant tunneling and light emittingdevices.

Additionally, a resonant tunneling structure could be formed by formingthe structure shown in FIG. 3, but by further forming another siliconlayer 250 over oxide layer 330. The device would be based uponelectrical current tunneling from Si layer 220 to layer 250, via theoxide layers and mediated by the Si quantum well 240.

Other resonant structures may be made by depositing additional oxide andSi layers 250, 340 and 260. If layers 250 and 240 are <20 nm thick, thena dual quantum well tunneling structure is produced and so on.

Thus, the inventors have grown an oxide/silicon multilayer structurethat is an epitaxial heterostructure. Such a plurality of interleavedlayers/structures can be used for novel devices.

For example, the inventive compound can be used for a gate dielectricfor a metal oxide semiconductor field effect transistor (MOSFET) bytaking silicon and growing an insulator on it, and then growing anepitaxial silicon layer on the insulator. If this epitaxial Si layerforms the channel of a transistor, then a silicon-on-insulator FET isformed.

Further, as alluded to above, the invention can be used to form asilicon on-insulator-based transistor with a gate dielectric on eitherside, or the inventive structures can be used for resonant tunnelingdevices (which are not transistors but are other devices) as brieflydescribed above.

Additionally, the inventive structure could be used for an opticaldevice in that the novel compound/silicon structure may be luminescent.

As an example of a multi-layer stack utilizing the structure of FIG. 3,with the Si layer 240 being less than 20 nm, a Si quantum well can beobtained, or a Si/oxide stack can be repeated, with the individual layerthickness being less than 5 nm, so that a super lattice structure thatacts as a luminescent material is obtained.

Further, it is noted that the present invention is not limited to amixed rare earth oxide of (La_(x)Y_(1-x))₂O₃. The present invention alsois equally applicable to other rare earth materials which could befitted into the same philosophy in terms of looking at the latticeconstants and matching them so that they could match silicon. Some othercandidate materials and oxides which could be used are samarium (e.g.,Sm_(x)Y_(1-x))₂O₃), cerium (Ce_(x)Y₁₋₀)₂O₃), Gadolinium(La_(x)Gd_(1-x))₂O₃), Gadolinium oxide and Europium oxide (e.g.,(Gd_(x)Eu_(1-x))₂O₃), etc.

With the unique and unobvious aspects of the invention, new compoundshave been developed in which as the yttrium oxide is growing a smallamount of lanthanum (or other rare earth material as noted above) isadded. The invention would find great benefit in microdevice structuringetc., due to its lattice matching with silicon. While the invention hasbeen described in terms of several preferred embodiments, those skilledin the art will recognize that the invention can be practiced withmodification within the spirit and scope of the appended claims.

1. A method of forming a semiconductor structure, comprising:epitaxially growing a first layer of mixed rare earth oxide on a siliconsubstrate, said mixed rare earth oxide being single crystal and latticematched to said silicon substrate; epitaxially growing a first siliconlayer on said layer of mixed rare earth oxide; epitaxially growing asecond layer of mixed rare earth oxide on the first silicon layer; andepitaxially growing a second silicon layer on said second layer of mixedrare earth oxide.
 2. The method of claim 1, wherein a lattice constantof said first and second layers of mixed rare earth oxide issubstantially a multiple of a lattice constant of silicon.
 3. The methodof claim 1, wherein said mixed rare earth oxide is single crystal, andcomprises a compound having a chemical formula (A_(x)B_(1-x))₂O₃,wherein A represents a first rare earth element and B represents asecond rare earth element.
 4. The method of claim 1, wherein said mixedrare earth oxide comprises a ternary mixed rare earth oxide.
 5. Themethod of claim 1, wherein said mixed rare earth oxide comprises a rareearth cubic ternary oxide.
 6. The method of claim 1, wherein said mixedrare earth oxide comprises (La_(x)Y_(1-x))₂O₃.
 7. The method of claim 1,wherein said mixed rare earth oxide comprises (Sm_(x)Y_(1-x))₂O₃.
 8. Themethod of claim 1, wherein said mixed rare earth oxide comprises(Ce_(x)Y_(1-x))₂O₃.
 9. The method of claim 1, wherein said mixed rareearth oxide comprises (La_(x)Gd_(1-x))₂O₃.
 10. The method of claim 1,wherein said mixed rare earth oxide comprises (Gd_(x)Eu_(1-x))₂O₃. 11.The method of claim 10, wherein 0.02<x<0.80.
 12. An optical device,comprising: a first layer of mixed rare earth oxide on a siliconsubstrate, said mixed rare earth oxide being single crystal and latticematched to said silicon substrate; a first silicon layer formed on saidlayer of mixed rare earth oxide; a second layer of mixed rare earthoxide formed on the first silicon layer; and a second silicon layerformed on said second layer of mixed rare earth oxide.
 13. The opticaldevice of claim 12, wherein a lattice constant of said mixed rare earthoxide is substantially a multiple of a lattice constant of silicon. 14.A light-emitting device, comprising: a first layer of mixed rare earthoxide on a silicon substrate, said mixed rare earth oxide being singlecrystal and lattice matched to said silicon substrate; a quantum wellcomprising a first silicon layer formed on said layer of mixed rareearth oxide; a second layer of mixed rare earth oxide formed on thefirst silicon layer; and a second silicon layer formed on said secondlayer of mixed rare earth oxide.
 15. The light-emitting device of claim14, wherein said first silicon layer has a thickness of less than 20 nm.16. The light-emitting device of claim 14, wherein said second siliconlayer has a thickness of less than 20 nm.
 17. The light-emitting deviceof claim 14, wherein a lattice constant of said first and second layersof mixed rare earth oxide is substantially a multiple of a latticeconstant of said first and second silicon layers.
 18. The light-emittingdevice of claim 14, wherein said first and second layers of mixed rareearth oxide and said first and second silicon layers have a thickness ofless than 5 nm, and form a super lattice structure that acts as aluminescent material.